In general, acrylonitrile-butadiene-styrene copolymers (ABS resins) have superior processability and graceful appearance properties as well as processability due to styrene, rigidity and chemical resistance due to acrylonitrile, and impact resistance due to butadiene rubber. Accordingly, acrylonitrile-butadiene-styrene copolymers are used in a variety of fields such as interior and exterior materials of vehicles, housings of home appliances, toys, etc.
In particular, in vehicle interior materials, etc., high thermal characteritics (e.g., heat deflection temperature (HDT)) are required. ABS resins to satisfy such thermal properties are generally prepared through emulsion polymerization or solution polymerization. In addition, an α-methylstyrene-acrylonitrile copolymer (AMSAN) having a high glass transition temperature (Tg) is melted with a rubber-reinforced graft copolymer (acrylonitrile-butadiene-styrene copolymer) prepared through emulsion polymerization within a predetermined rubber content range, and mixed with the same. In this case, in order to further enhance heat resistance, upon preparation of the rubber-reinforced graft copolymer, α-methylstyrene having a higher glass transition temperature may be used instead of styrene. However, when α-methylstyrene is used, a polymerization rate is dramatically decreased by low reactivity and polymerization stability decrease of α-methyl styrene in an emulsion polymerization step, and solid generation amount increases, whereby productivity and heat deflection temperature are decreased.